This invention relates to travelling wave tubes and in particular to coupled cavity travelling wave tubes.
Coupled cavity travelling wave tubes are commonly formed with coupling plates defining the ends of each cavity which act also as ferro-magnetic pole pieces.
It is important that the coupling plates/pole pieces exhibit good heat conduction in order that the heat generated in the region of the beam coupling hole in each be conducted away. However, the materials which must be used for their magnetic function (e.g. iron) are not generally ideal heat conductors and for this reason it is common to provide some means of enhancing heat conduction from the beam coupling hole outwardly. Such means as presently known include the use of a copper insert in the pole piece or the formation of the pole piece by an iron-copper-iron laminate.
Another approach is to provide a water passage through the coupling plate/pole piece. This may be achieved as illustrated in FIG. 1 of the accompanying drawings.
Referring to FIG. 1, this shows, part brokenaway, a section through a coupling plate/pole piece taken transversely of the tube axis 1. The coupling plate or pole piece consists of a circular disc 2 having a central beam hole 3. In this example the beam hole 3 is surrounded by a drift tube 4 as known per se. The disc 2 is formed in two parts, both of iron, one part referenced 5 in which a water channel 6 is formed in its surface and the other part, referenced 7 being provided to act as a closure for the water channel 6. Water manifolding, not shown, is provided at convenient locations in order to enable water, or of course other coolants, to be passed through the channel 6. Viewed in the direction of the axis 1, water channel 6 would be arcuate in shape.
Because of the corrosive effects of water passing through channel 6 it is necessary to protect the iron surfaces of the parts 5 and 7 defining the water passage 6. Typically therefore the facing surfaces at least of the parts 5 and 6 would be electro-plated (e.g. with nickel) but, because of the recessed nature of the channel portion formed in the part 5 the use of an electroless plating process is called for.
In order to improve the temperature distribution around the inner surface of the beam hole 3, a cylindrical copper liner represented in dashed outline at 8 in FIG. 1 is sometimes provided. The liner 8 tends to provide compensation for the heat conduction distorting effects of the impedance to heat conduction presented by the normally provided coupling slot which is not shown in FIG. 1 since it is located beyond the point at which the disc 2 is shown broken away. The coupling hole referred to will be similar to that represented at 12 in FIG. 3, to be described later. As will be appreciated, this impedance effects one sector of the disc 2 rather than the disc uniformly.